Carbon Electrode for use in aqueous electrochemical devices and method of preparation

ABSTRACT

A high tensile strength, highly conductive flexible composite sheet consisting of fibrillated PTFE polymers, performance enhancing dopants, and carbon particles useful in electrochemical devices such as flow through capacitors, capacitive deionization, electronic water purification, fuel cells, capacitors, super-capacitors and a method of preparation. The method consists of mixing the materials and calendering to the desired thickness and strength all at room temperature and leaving the material in a wet state without further drying.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a division of U.S. patent application Ser. No.10/921,673, filed Aug. 8, 2004, the disclosure of which is herebyincorporated by reference herein, which claims the benefit of U.S.Provisional Patent Application Ser. No. 60/496,341, filed Aug. 19, 2003,by the present inventors.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to carbon electrodes made with fibrillatedpolytetrafluorethylene (PTFE) for electrochemical devices and theirmethods of preparation.

2. Prior Art

The process of fibrillating PTFE has been known for some time and hasbeen included in many patents that will be discussed later. Duringfibrillation, other ingredients are added that provide various aspectsof functionality such as carbon for electrical capacitance and chemicaladsorption.

The typical process to produce fibrillated PTFE with carbon is to mixPTFE, carbon, and solvents under high shear and high temperature,biaxially calender the material at high temperature, extrude into thefinal form at high temperature, and dry the product at high temperatureto remove the solvents. It is clear that high temperature was considereda critical part of the process of creating PTFE carbon electrodes in theprior art and that the final product must be dried prior to use indevices. Any solvents that are critical to the operation of the devicemust be added in a subsequent operation.

It is clear from the prior art for producing carbon containingelectrodes made with PTFE that the first step must be a high shearmixing step. Ree et al. (U.S. Pat. No. 4,153,661), in describingproduction of carbon filled PFTE webs, specifies to mix materialstogether and then high shear mixed between 50-100 C prior to biaxialcalendering. Solomon (U.S. Pat. No. 4,379,772) shows that materials areto be mixed without solvents, and then high shear mixed to causefibrillation. Shia (U.S. Pat. No. 4,556,618), describes the importanceof initial high shear mixing on the final properties of the carbonelectrode. Finally Andelman (U.S. Pat. No. 6,127,474), while describingthe manufacture of carbon electrodes for a flow through capacitor,specifically states that the ingredients are typically mixed under highheat and high pressure conditions.

The next step in the process is the biaxial calendering of the mixedmaterial to produce a fibrillated sheet. A consistent theme in the priorart is the insistence of high temperature during this process. Ree etal. (U.S. Pat. No. 4,153,661) teaches that this step should be done from50-100 C. Solomon (U.S. Pat. No. 4,379,772) recommends a targetcalendering temperature of 50 C. Andelman (U.S. Pat. No. 6,127,474)recommends 320 F as a target temperature. The purpose of the biaxialcalendaring step is to fibrillate the material and produce a flexiblesheet. Fibrillation occurs when the material is subjected to high shear,which causes the microscopic particles of PTFE contained in the mixtureto unravel. These particles unravel similar to a ball of yarn that hasmany strings wrapped around it. As the many pieces of string of the manyparticles unravel, they begin to intertwine with each other, creatingthe microscopic web that holds together all of the particles andprovides the sheet with tensile strength and dimensional stability. Theshear rate of an operation is significantly affected by the temperatureof the material being sheared. As the temperature rises, the viscosityof the material drops and the shear rate drops. By biaxially calenderingmaterial at higher temperatures, the time and effort to produce anequivalently fibrillated sheet is much longer. Also, running thisoperation at higher temperatures, especially those approaching theboiling point of water, causes the material to lose water quickly. Aswater is lost, the viscosity of the material rises in an uncontrolledmanner, the rate of fibrillation increases quickly, and makes it verydifficult to fibrillate to a consistent level.

The drying step is universally recommended in all prior art references.When carbon PTFE material is dried, water that had been incorporatedinto the very small pores within and around the carbon particles isremove as vapor. When the material is rewet, some of these originallywet internal pores do not rewet. Since these devices work on theprincipal of ion absorption onto surfaces of carbon particles, the lesssurface available to the ion, the less absorption. When the absorptionof water is reduced, the overall capacitance of the device is reduced.Also, the electrical resistance of the device increases. This is due tothe lack of open pathways for the ions to electrochemically diffuse intoand out of the carbon electrodes. When this electrochemical diffusionresistance is higher, the speed at which the device can operate isreduced, thereby reducing the ion removing power. In summary, drying thematerial reduces the capacity of the device by 10 to 15%.

Vallance (U.S. Pat. No. 3,890,417), Goldsmith (U.S. Pat. No. 3,281,511),Ree et. al. (U.S. Pat. No. 4,153,662), Bernstein (U.S. Pat. No.4,320,185), Solomon (U.S. Pat. No. 4,379,772), Shia (U.S. Pat. No.4,556,618), Morimoto (U.S. Pat. No. 4,862,328), Hiraksutka (U.S. Pat.No. 6,072,692), and Andelman (U.S. Pat. No. 6,127,474) all describe theimportance of drying carbon electrode material. There are someapplications where it is important to eliminate any traces of water dueto the design of the device and others where the PTFE or otherbinder/component must undergo sintering at high temperature. Indescriptions for electrode material clearly destined for aqueouselectrochemical devices, drying is a described as a key step. Ree et al.(U.S. Pat. No. 4,153,661) recommends drying at 20-100 C for anywherefrom 1 to 100 hours. Andelman (U.S. Pat. No. 6,127,474), describing theprocess to produce carbon electrodes for use in aqueous electrochemicaldevices, states as examples either formulations that do not have anywater, or small amounts that then are subjected to extremely hightemperatures. In any case, the resultant carbon electrodes produced bythe Andelman (U.S. Pat. No. 6,127,474) patent are dry.

Not only do dried electrodes never regain their original ionic capacity,they also take an extremely long time to rewet. Studies have shown thatit takes anywhere from a few days to a few weeks for a dried electrodeto come to equilibrium. The equilibrium level is not equivalent to theoriginal absorption capacity as mentioned above.

It is critical to prevent active surface area from drying due to theelectrode's inability to rewet and hence loss performance. In order forthe wetted surfaces to function in a device, ions must be originallypresent in the water that fills the porosity. If this water does notcontain a sufficient amount of ions or conductivity, upon assembly, theionic capacity of the device is reduced an can never recover. The reasonthat ions can not diffuse into these deionized areas after assembly isdue to the fact that the carbon electrode is sandwiched in between asolid current collector and an ion specific membrane in most aqueouselectrochemical devices. This arrangement prevents any further ion pairsfrom diffusing into the electrode, hence reducing the ionic capacity ofthe electrode and device.

OBJECTS AND ADVANTAGES

Accordingly, several objects and advantages of our invention are:

-   -   a) the elimination of performing any of the process steps at        temperatures above room temperature;    -   b) the elimination of the high shear mixing step;    -   c) incorporation of performance enhancing dopants;    -   d) the complete elimination of the final drying step; and    -   e) how these advantages drastically improve the overall final        performance of the device.

In this invention, we have shown that mixing should be done at roomtemperature and via distributive mixing versus shear mixing.Distributive mixing slowly moves small portions of the material from onespot in the mixer to another. After this happens thousands of times, thebatch appears homogeneous on a microscopic level. This greatlysimplifies this first operation by shortening the cycle time, reducingthe capital cost of equipment since only a pastry mixer is necessary,and reduces the process variability. When a high shear process isutilized, it is very critical that the degree of shear is accuratelycontrolled or the product properties will vary significantly. The levelof shear imparted to the material is extremely sensitive mixing speed,blade clearances, temperature, and water content. If any of theseparameters vary slightly, the product quality will change significantly.The ability to eliminate the high shear mixing step provides a drasticimprovement for process control and simplicity.

The next process step is biaxial calendering which performs thefibrillation process. Conducting this process at room temperature alsocreates an easier, faster, and more consistent process by reducing thetime, eliminating heaters and temperature controllers, and bettercontrolling the viscosity of the material. By running at roomtemperature, the evaporation rate of water from the material beingfibrillated is very low, which allows for tight control of the finalfibrillation level.

Testing has shown that process described in this patent producesmaterial with tensile strength in the range of 0.7 MPa (0.07 kg/mm),more than sufficient for use in automated equipment and device assembly.Morimoto states desired values of at least 0.02 kg/mm (0.2 MPa) forcarbon electrodes use in capacitors. Andelman states that the tensilestrength of acceptable material for use in a flow through capacitorshould be greater than 0.05 kg/mm2.

The last step of drying the material is skipped in this invention. Byskipping the drying step, not only is the process simplified and greatlyshortened by weeks to hours, the product quality for use in an aqueouselectrochemical device such as a flow through capacitor, electronicwater purifier, capacitive deionization device, or other aqueous basedcapacitors is greatly improved. The equivalent series resistance (ESR)of an electronic water purifier cell made with non-dried electrodes is25% less than one made with previously dried electrodes (fromapproximately 0.25 to 0.19). This translates into a 10-15% increase inaverage purification, or from an average of 60% to 68%. This allows for10-15% less equipment or cells to be used to accomplish the samepurification and flow rate.

This invention proposes the incorporation of a dopant such as salt intothe initial formulation in levels equal to or greater than the operatingconcentrations of the device. The dopant chosen should be appropriate tothe device and application. In the case of water purification, thedopant can be sodium chloride, potassium chloride, or other appropriatealkali metal halides. By incorporating the salt into the initial formulaand skipping the drying step, wetted internal porosity contains ionsthat will contribute to the capacity of the device. The addition of saleinto the formulation decreases the electrical resistance by another 25%(from approximately 0.18 to 0.13) and increases the average purificationby another 10-15% from 68% to 75%.

The overall effect of skipping the drying step and incorporating saltsinto the formulation is 50% reduction in ESR and 25% increase in overallpercent purification. The reduction in ESR also translates into anequivalent reduction in electrical consumption which is a very importantattribute for customers installing water purification equipment inremote locations.

In summary, these novel and unobvious inventions greatly simplify andimprove the preparation methods and performance of aqueouselectrochemical devices by reducing the complexity of the equipment,reducing process variability, reducing cycle time, and reducing theelectrical resistance of the material which improves the capacity of thedevice.

Further objects and advantages of our invention will become apparentfrom a consideration of the drawings and ensuing description.

SUMMARY OF THE INVENTION

This invention describes an improved PTFE fibrillated carbon sheetelectrode for use in aqueous electrochemical devices and a simplifiedprocess for producing the electrodes.

The method consists of mixing PTFE, various types of carbon, salts, andwater together under distributive mixing conditions at room temperatureand fibrillating the material by biaxial calendering at room temperatureto the desired strength and thickness levels. The material is left in awet condition so as to enhance the electrochemical properties.

DETAILED DESCRIPTION OF THE INVENTION

The PTFE sheet material of the invention is prepared by mixing PTFEdispersion, carbon black, activated carbon, water, and a salt togetherin a mixer. The mixer can be a single planetary mixer with one blade, adouble planetary mixer with two blades, or any other type of mixer thatcan distributively mix the materials without any significant amounts ofshearing. The key aspect of this step is to distribute the ingredientsas much as possible without shearing the material or increasingevaporation rate of water. The batch size can be from 1 pint up tohundreds of gallons of volume of material.

Once the composition of carbon, water, sale, and PTFE is adequatelydispersed, it is removed form the mixing container and biaxiallycalendered to the desired thickness and fibrillation level. The calendaris a set of two large rollers spinning in opposite directions with asmall gap in between the rollers called a nip. The material is placed atthe nip on the side which will allow the material to be pulled into thenip by the rollers. The material is removed, folded, reinserted into thenip. This is repeated until the sheet material has fibrillatedsufficiently.

EXAMPLE 1

The product is mixed at 10-60 rpm depending on the size of the mixer ina single or double planetary mixer with ceramic coated or stainlesssteel blades. Mixers that provide acceptable distributive mixer areHobart and Kitchen Aid. The mixing container is usually made fromstainless steel or coated carbon steel. The preferred formulation isActivated Carbon (Nuchar RGC, average particle size of 15 microns,surface area of 1800 m2/gm) 4,690 grams, Carbon Black (Black Pearls2000, surface area of 1,500 m2/gm) 469 grams, Teflon T-30 dispersion 969grams (60% solids), DI water 13,192 grams, and the alkali metal halidesodium chloride 68 grams. All of the carbon black, T-30, salt, and waterare added to the mixer and mixed at room temperature and low speed for15 minutes. Then 1,100 grams of the activated carbon is added and mixedfor 5 minutes. The activated carbon is added in this size incrementuntil complete. Then the batch is mixed for the last time for 10 minutesand removed from the mixer. At this point, the water content of theresultant material is approximately 68%, or the ratio of water to drycomponents is approximately 2 to 1.

The mixed batch is then run through a calender such as an Acme RollSheeter. The nip on the rollers is set to 0.030″ and speed set atapproximately 200 rpm. A portion of the material, typically 300 grams isput through the nip and received on the return conveyor. The material isrecovered, rotated 90 degrees, folded in half, and reinserted into thenip of the calender. This process is repeated until the material haspassed through the nip 10-20 times. Room temperature is acceptable forthe process and calender rollers. By the end of this process, thematerial thickness should be in the 0.040″-0.125″ range and is ready tobe sheeted. A typical thickness is 0.050″.

The product from the calender is trimmed to the desired width. Theroller speed is reduced to 30-90 rpm with a typical speed of 45 rpm. Thenip size is set to 0.005″ so as to produce a product in the0.008″-0.012″ range. The material is fed one last time through the nipto achieve the desired sheet thickness. As the material exits thecalender, it is placed into a sealable container, a container with soaksolution, or assembled directly into the electrochemical device. Thisoperation can also be done with a second calender already set to thecorrect speed and nip size thereby streamlining the production of theelectrode.

EXAMPLE 2

The same procedure is followed as in example one. The activated carbonis substituted with Maxsorb Activated Carbon from the Kansai Cokeproduct.

EXAMPLE 3

The same procedure is followed as in example one. The ratio of activatedcarbon and carbon black to dry Teflon is reduced from 9:1 to 8:2.

EXAMPLE 4

The same procedure is followed as in example one. The dopant sodiumchloride is substituted with another alkali metal halide potassiumiodide.

EXAMPLE 5

The same procedure is followed as in example one. The Teflon T-30dispersion is substitute in part or whole with dry Teflon 601A.

1. A method of preparing a high tensile strength highly conductiveflexible composite sheet material for use as an electrode in anelectrochemical device, comprising the steps of: a. distributive mixingof polytetrafluoroethylene, carbon, dopants, and solvent under low shearconditions and b. fibrillating said mixture by biaxial calendering todesired strength and thickness whereby said electrodes can be assembleddirectly into said device without drying.
 2. The method of claim 1wherein said carbon consists of 8 parts activated carbon to 1 partcarbon black.
 3. The method of claim 1 wherein said electrode consistsof 9 parts total carbon to 1 part polytetrafluoroethylene.
 4. The methodof claim 1 wherein said dopant is chosen from one of the many alkalihalides.
 5. The method of claim 1 wherein distributive mixing isperformed by a planetary mixer.
 6. The method of claim 1 furtherincluding a means to stabilize the water content of said electrode. 7.The method of claim 1 wherein said biaxial calender has a means forrotating said electrode material and reintroducing into said biaxialcalendar.
 8. The method of claim 1 further including an operation toreduce the thickness of the carbon sheet.